The present invention relates generally to ducts of the type currently employed for guiding electric conductors in switch or control boxes, that is, boxes or cabinets for accommodating switch gear or control gear.
Such ducts which are for example made of plastic typically have a U-shaped cross section and along the center line of their endwalls usually have uniformly spaced-apart openings for receiving fastening members necessary for securement to some sort of a support member. These openings are usually longitudinally elongated slots for enabling the adjustment of the position of the duct relative to the support member on which it is to be mounted.
The present invention relates more particularly to the securement of such a duct on a support member which is previously installed in the switch or control box. Conventionally such a support member comprises a perforate plate generally made of metal. It therefore follows that in order to install the ducts a control box must usually be equipped with such a perforate plate.
In addition to control boxes fitted with such perforate plates there are control boxes equipped with a frame comprising an array of support sections, generally made of metal, and of U-shaped cross section, the legs of sidewalls having right-angle flanges or return bends for snapping on the desired switchgear or control gear.
According to standardized arrangements the legs may be of different height and their right angle flanges are usually outwardly directed, away from each other, and at the same level. As a variant such legs are at different heights, and their right angle flanges are then usually inwardly directed toward each other, but at different levels.
In any event heretofore such ducts could not be easily mounted on such a support section.
Moreover the securement of ducts to a perforate plate has usually required the use of screws or even rivets. Such fastening members must be picked out of a batch of screws or rivets loose in a container which is time-consuming and a source of losses of such screws or rivets.
When the fastening members are threaded fasteners or screws, whether of metal or plastic, after passing-through the endwall of the duct and the perforate plate they must receive a nut from the remote side of the perforate plate. Since this requires the electrician or installer to work from the rear of the perforate plate it is in practice inconvenient and relatively time-consuming.
Further, as regards metal threaded fasteners, their use is contrary to safety regulations since they form a conductive link between the electric conductors in the duct and the subjacent perforate plate, to the detriment of the normal uninterrupted insulation of the endwall of the duct.
As for the use of rivets, their use requires special tools whether the rivets are made of metal or plastic.